TM 5-6635-386-12&P the  counts  will  equal  the  square  root  of  the average count or mean. Given One   standard   deviation   equals   68%   of   the population of events for a series. Put these all together and: + 1 Std Dev = + - = Averagecount 68%Prob. This  finding  can  be  put  to  good  use  in  evaluating  tester performance  if  we  use  it  to  inspect  a  series  of  counts taken in the standard count mode. (The procedure may also be used to evaluate any series of  nuclear  tester  counts.    It  is  only  necessary  that  the tester not be moved during the series of counts.  Counts could  be  taken  in  any  operating  position,  with  another tester nearby, etc.). Two evaluation methods will be described. One  uses  a  hand  calculator  with  statistic  functions,  the other uses a manual checkoff method.  Both techniques work  well.    Both  should  be  used  with  ten   successive counts taken in the 1/4 minute time period.  (This will not work if counts are taken in other time periods due to the averaging   or   normalizing   of   the   tester   on   long   time periods). 1-10. CALCULATOR EVALUATION USING STANDARD DEVIATION Enter the series of numbers into the calculator following calculator instructions for standard deviation.  Determine the   deviation   and   place   it   in   one   of   the   calculator’s memories. Pull up the mean  of  the  series  of  counts  and  determine the square root of the mean. Divide the deviation by the square root. From  the  above  discussion,  if  the  series  was  "perfect", the   square   root   would   equal   the   deviation   and    the division would be "1.0".  The series will seldom work out perfectly,  however,  and  some  variance  from  1.0  will  be observed.    It  is  normal  for  this  to  lie  between  0.75  and 1.25  with  a  general  tendency  towards  1.0  for  a  normal tester. 1-11.  MANUAL EVALUATION Take  the  same  series  of  ten  numbers.    Use  1/4  minute time key. If  the  standard  deviation   of   the   series   is   to   lie   within + - average    and  this  is  to  include  68%  of  the  series, then   this   is   merely   stating   that   68%   of   the   numbers should lie within + - of the average or mean and that 32% should be outside the average or mean. That is, out of a series of ten numbers, 32% or about 3 out of 10 will lie outside plus or minus the square root of the average. We  simply  add  up  a  series  of  numbers,  average  them, determine  the  square  root  of  the  average  and  then  add and subtract this square root to the average. The resultant high and low limits will include 68% of the numbers  in  the  series  and  32%  will  be  higher  or  lower than the limits. The  following  example  illustrates  a  typical  tester  placed in  Standard  Count  configuration  and  using  the  Student Field  Data  Worksheet  (DA  Form  5448R  (Moisture  and Density  Tester  Field  Data  Worksheet)),  Figure  1-3,  to accumulate the data for ten counts.  DA Form 5448-R is located   at   the   back   of   this   TM   for   local   reproduction authority.  This tester was normal in all respects. We seldom are fortunate enough to have the series work out   exactly   3   out   of   10   each   time   we   run   a   series. However,    if    we    make    an    intelligent    allowance    for variations in numbers we will observe that the series will exhibit    a    general    trend    towards    3    out    of    10.        An occasional    2    out    of    10    will    be    observed    and    an occasional  4  out  of  10  will  be  observed.    However,  the statistic probability of a 5 out of 10 or a 1 out of 10 is very slim,  and  such  splits  should  be  very  rare  for  a  normal tester. It  is  important  to  observe  the  average  from  one  such Standard Count Evaluation to another.  If the average is within approximately 1/3 to 112 of the square root of the prior average, then the difference between the two series is normal. 1-10